Experimental approaches to understand the role of genetic and environmental influences on the microbial community associated with "Daphnia"

Every multicellular organism on this planet is associated with a diverse community of microorganisms referred to as its microbiota. In recent years, myriads of effects resulting from this interaction have been revealed. The series of experiments condensed in this thesis aimed at investigating this relationship in more detail in planktonic crustaceans from the genus Daphnia and in particular the water flea Daphnia magna. The findings from these experiments provide important basic knowledge essential for future host-microbiota related research in this system.
Part one of the thesis comprises all experiments conducted by myself. In the first chapter I reciprocally transplanted the microbiota between the two species Daphnia magna and Daphnia pulex to examine the degree of specificity in these associations and determine the effects on host fitness as a consequence thereof. We found that being associated with a foreign microbial community did not negatively affect maturity, fecundity and size in both Daphnia species, letting us speculate that the two partners did not adapt to each other. This hypothesis was further supported by the insights gained when investigating the transmission of the whole microbial community, which was the purpose of the second chapter. Compositional comparisons between maternal and offspring microbiota revealed a stable fraction of microbes that gets transmitted, neither influenced by the origin nor the diversity of the microbial community.
The third chapter of the thesis aimed at assessing the stability of the Daphnia-microbiota association and to what degree established microbial communities were still modifiable. We paired and raised juveniles from three different locations (Belgium, Germany and Switzerland) in a full factorial design and compared their microbiota. We found that the microbiota is easily modifiable and that microbial communities greatly influence each other’s composition. In addition, the results allowed the conclusion that the microbiota is not genetically controlled. We further investigated this in the fourth chapter dedicated to evaluate if the homozygosity level of the host influences microbial diversity by comparing the microbiota of selfed and outcrossed animals. From the results we conclude that the host genotype has little influence on the diversity of the microbiota.
Taken together, these results suggest that although microbial communities play a crucial role in Daphnia, the two partners did not adapt to one another leading us to propose a scenario of how transmission in the Daphnia-microbiota association might take place.
Part two of the thesis encompasses work resulting from collaborations. In chapter five, Alexandra Mushegian tested the role of bacteria for animal functioning, showing a positive effect on embryonic development under warm temperature conditions.
The goal of the last chapter, conducted by Karen Sullam, was to investigate the effect of temperature, host clone, and their interaction on host-associated microbiota. The experiment showed that the interaction of the factors affected microbial community structure while their diversity was more affected by host clonal background.